Sex matters. Being born a girl or boy carries implications for health and illness. Despite considerable interest in the development and maintenance of sex-related processes, there are noticeable gaps in the literature that we propose to address. We will study how the interplay between biological predispositions and social experiences shapes gender development at the adolescent transition;sex-related characteristics increase in importance at this time, with longterm consequences for opportunities, choices, and health. Taking a biopsychosocial perspective, we will study family socialization in 120 girls exposed to varying levels of androgen during prenatal development owing to congenital adrenal hyperplasia (CAH);we will compare them to a previously-assessed group of 150 typical girls. The aims are: (a) to chart how gendered characteristics (identity, interests, personal-social attributes, attitudes, activities) are linked to family socialization and how the links are moderated by degree of prenatal hormone exposure and by parent-child relationship quality;and (b) to examine how gender development is linked to psychological health (self-concept and internalizing and externalizing problems). Girls aged 11-13 will be interviewed about their gendered characteristics, psychological health, and relationships with their parents;they will provide DNA via saliva for analysis of mutations in the gene causing CAH (reflecting degree of androgen exposure). During seven follow-up phone calls, girls will describe their gendered activities and companions that day. Parents will provide information about their gender socialization through questionnaires and reports during four phone calls about their own gendered characteristics and activities, expectations and beliefs for their daughter, management of their child's gendered activities, and about warmth in the parentchild relationship. As a natural experiment, girls with CAH provide a magnification of normal developmental processes;combining girls with CAH and typical girls yields a sample with variation in both hormone exposure and family socialization, and sufficient statistical power for hypothesis-testing. The work has public health impact, showing how gender matters for behavior and health, and how biological predispositions affect socialization experiences. This information can lead to interventions to improve mental and physical health. Data also will aid in management of children born with ambiguous genitalia and children with gender dysphoria.